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Big Bang. Conceptual computer artwork representing the origin of the universe. The term Big Bang describes the initial expansion of all the matter in the universe from an infinitely compact state 12-15 billion years ago. The initial conditions are not known, but less than a second after the beginning, temperatures were trillions of degrees Celsius and the primordial universe was much smaller than an atom. It has been expanding and cooling ever since. Matter formed and coalesced into the galaxies, which are moving away from each other as they are carried by the expanding universe. Background radiation in the universe is considered a remnant of the Big Bang.